Research is proposed on factors that modify sensitive periods for learning. Special foci are hormonal correlates of variations in readiness to learn at different stages of life, modifiability of sensitive learning periods by various hormonal treatments, and effects of environmental deprivation on delaying sensitive period closure. Avian vocal learning is used as the paradigm. There are correlations between fluctuations in the levels of steroid hormones in the blood serum during the first year of life, and the time when learning occurs most readily. These provide the basis for experiments in which castration early in life will be followed by various regimes of hormone therapy. Testosterone and estradiol will be used, both separately and in combination, as the best current prospects for a role in learning. The potential lability of sensitive periods will be explored by behavioral and hormonal manipulations. Subjects will be two bird species extensively studied in prior work, with different patterns of song structure and development. Radioimmunoassay techniques will be used to track variations in blood hormone levels. New techniques will be applied for automated computer analysis of recorded vocalizations. These provide major improvements in quantitative description of vocalizations, and tracking changes in the stability of vocal patterns as development procedes. Learning of both optimal and suboptimal stimuli will be studied, including the role of innate responsiveness to key elements which have "enabling" effects, and facilitate learning of suboptimal stimuli by their presence. A major effort will be made to reinstate vocal plasticity after closure of the sensitive learning period. Success in this endeavor will provide new insights into conditions under which neural plasticity can be reinstated at a time when the threshold for acquisition of new material is normally high. Sensitive periods are a focus of research in many aspects of developmental neurobiology, and parallels with imprinting and song learning are providing fruitful common ground with ethologists, psychiatrists and psychologists concerned with normal and abnormal human development. The results will have implications for understanding the impact on behavioral and neural development of restricted access to certain kinds of stimulation at particular stages of development.